It is difficult to finish a high, that is substantially 50/50 or greater, nylon/cotton fabric with a flame retardant and have acceptable flame resistance properties coupled with durability after multiple washings. One flame retardant process suitable only for cotton fibers which provides satisfactory and durable flame resistance, known as the PROBAN process, consists of treating the cotton fabric with a prepolymer of tetrakis-(hydroxymethyl)phosphonium salt and urea, followed by ammoniation (THP/urea-precondensate/ammonia). The PROBAN process, licensed by Albright & Wilson, is described in the following U.S. Pat. Nos.: 4,078,101; 4,145,463; 4,311,855; and 4,494,951, all to Albright & Wilson, the disclosures of which are hereby incorporated by reference to the extent necessary to explain the THP salt/urea precondensate process. See also U.S. Pat. No. 4,346,031 to Elgal et al. This process is considered effective and is widely promoted by at least two companies for imparting flame resistance to 100% cotton fabrics; it is not promoted or advertised for polyester/cotton blends or nylon/cotton blends.
The amount of flame retardant chemical that would be present for different nylon/cotton blends can be calculated. For example, assume that THF/urea-precondensate/ammonia was used on 100% cotton fibers and that 3 wt.% phosphorus was fixed. This amount of phosphorus would be above the amount that would be expected to be required for good flame resistance of cotton fibers alone. However, if these flame-retardant-treated cotton fibers were then blended with nylon fibers to obtain a 55/45 nylon/cotton fabric, the "effective" phosphorus concentration of the blend would be only about 1.3 wt.%. Thus, a PROBAN-type process, in which only the cotton is treated, might be used successfully with lower nylon/cotton blends containing only a small amount of nylon up to 15/85, where such a process would result in an "effective" phosphorus concentration of the 15/85 nylon/cotton blend of about 2.4 wt.%. This concentration is adequate for good flame resistance.
It has been observed that if other compatible flame retardants, such as the cyclic phosphonate esters, as exemplified by Antiblaze 19 (identified in more detail below), are applied to the nylon fibers of a higher-nylon nylon/cotton blend, a satisfactory level of flame resistance can be obtained initially. However, the flame resistance is substantially reduced after about 15 launderings, because the cyclic phosphonate esters are not substantive to the nylon. Therefore, it was recognized that flame retardant chemicals for nylon fibers must somehow be made to perform better in order to produce a flame resistant high-nylon nylon/cotton blend fabric that retains enough total phosphorus for good flame resistance after multiple washings, thereby providing both satisfactory flame resistance and durability for a nylon/cotton blend fabric.
One such improved flame retardant system for nylon/cotton fabrics was developed using a one-step, two component system, one component for the nylon fibers and the other for the cotton fibers. The process, as described in copending application Ser. No. 911,729, consists of treating the nylon/cotton fabric with a finish containing both Antiblaze 19, for the nylon fibers, and tetrakis-(hydroxymethyl)phosphonium sulfate and urea (THPS/urea), for the cotton fibers. Flame resistant nylon/cotton fabrics prepared by this process showed improved flame resistance durability as compared with applying the THPS/urea and Antiblaze 19 separately as described in commonly-assigned application Ser. No. 870,892 filed June 5, 1986. However, it was not possible to obtain optimum flame resistance, durability, and an acceptable hand by this process for nylon/cotton blends containing more than 40% nylon.
Thus, with these processes, flame resistance of the fabric was adequate when tested, assessed by FTM 5903, initially, but was reduced to an unsatisfactory level after 15 or so washings either because the amount of phosphorus which can be fixed on the blend by the flame retardant process was limited or because the flame resistance durability or the hand was not adequate for nylon/cotton blends containing more than 40% nylon.